The present invention relates to a method for the simultaneous modernisation of a plant for ammonia production and of a plant for urea production.
More specifically, the invention relates to the simultaneous modernisation of a plant for ammonia production of the type comprising a section for the production of raw ammonia synthesis gas, which comprises carbon monoxide, hydrogen and nitrogen, a carbon monoxide conversion section, a decarbonation section, a methanation section, a compression section of the ammonia synthesis gas and an ammonia synthesis section, provided in series, and a plant for urea production of the type comprising a carbon dioxide compression section, a urea synthesis section and a urea recovery section, provided in series.
In the following description and enclosed claims, by the term xe2x80x9cmodernisationxe2x80x9d, it is intended to mean the modification of a pre-existing plant with the purpose of improving its performance and obtaining, for example, an increase of the production capacity and/or of the conversion yield as well as a reduction of the energy consumption.
In particular, in this case, by the term xe2x80x9csimultaneous modernisationxe2x80x9d, it is intended to mean a modernisation that concerns at the same time both an existing plant for ammonia production and an existing plant for urea production, in order to increase the production capacity thereof, while maintaining the main equipment of the high pressure synthesis sections (synthesis loop) as well as of the decarbonation, methanation and compression sections in general.
According to a further aspect thereof, the present invention relates also to a process for the combined production of ammonia and urea as well as to a plant for implementing such process.
The present invention has specific application in those cases wherein the ammonia plant and the urea plant are intimately correlated between each other, that is when all or anyway the greatest portion of the ammonia produced is converted into urea, making it react with the carbon dioxide obtained as by-product in the preparation of the ammonia synthesis gas.
As known, with respect to the production of ammonia and urea, the need of having on one hand plants of ever increasing capacity and operation efficiency and on the other hand ever lower investment and operation costs as well as lower energy consumption, is felt more and more.
To this end, methods have been proposed in the field, for the modernisation of existing plants both for ammonia and urea production substantially based on the modification of the synthesis reactor, on the replacement of the apparatuses upstream and/or downstream of the synthesis reactor with apparatuses of greater capacity and/or on the additional provision of new apparatuses in parallel to the existing apparatuses.
For example, in EP-0 202 454 a method is disclosed for the modernisation of a reactor for ammonia synthesis, whose catalytic beds are transformed from the axial type to the axial/radial type with an ensuing increase of the conversion yield of the reactor and therefore of the production capacity of the ammonia plant.
On the other hand, in EP-A-0 796 244 a method of modernisation of a plant for urea production is disclosed, which foresees the addition of a partial decomposition step of the carbamate in aqueous solution recycled to the synthesis reactor. With this method of modernisation, it is possible to remarkably reduce the amount of water recycled to the synthesis reactor, thus permitting to obtain an increase of the conversion yield and therefore of the production capacity of the plant.
One of the main problems that is encountered when a simultaneous increase of production capacity is considered in existing plants for ammonia and urea production, which are correlated to each other, is that of increasing accordingly the capacity of the sections upstream of the respective synthesis reactors.
In particular, the decarbonation, methanation and compression section of the ammonia synthesis gas as well as the compressors of the refrigeration cycles within the synthesis loop of ammonia and the compression section of the flow comprising carbon dioxide fed to the urea synthesis section are bottlenecks for the capacity increases, in that they are rapidly overloaded by the unavoidable increases of the flow rates of the reactants.
This problem is more serious when the existing plants have already being submitted to modernization according to the prior art and therefore the aforementioned sections or apparatuses are already at their operating limits.
In these cases, the methods of modernization according to the prior art only propose the replacement of the existing apparatuses with apparatuses of greater capacity or the addition in parallel of new apparatuses to the existing apparatuses, so as to increase the overall capacity of the various sections upstream of the respective synthesis reactors.
These provisions have a very bad impact both on the investment costs and on the energy consumption; furthermore the implementation of the methods of modernisation according to the prior art can be very complex.
Notwithstanding the ever increasing interest in the industry of modifying the existing plantsxe2x80x94instead of realising new plantsxe2x80x94in order to increase the production capacity and decrease the energy consumption with minimum investments, because of the aforesaid disadvantages, the modernisation of the plants for ammonia and urea production implies to date high investments, sometimes even comparable to those required for building new plants. Furthermore, with the methods of modernisation according to the prior art, the increase of production capacity is generally achieved to prejudice of the conversion yield, and therefore with greater energy consumption.
The technical problem underlying the present invention is that of providing a method for the simultaneous modernisation of a plant for ammonia production and a plant for urea production that provides for an increase of the respective production capacities and implies low energy consumption at low investment costs and is technically easy to implement.
According to the present invention, this problems is solved by a method of the above indicated type that is characterised in that it comprises the steps of:
providing a carbamate synthesis section and a carbamate decomposition section;
providing means for feeding a suitably compressed raw ammonia synthesis gas flow comprising carbon dioxide, hydrogen and nitrogen to the carbamate synthesis section;
providing means for feeding a portion of a flow comprising ammonia, hydrogen and nitrogen obtained in the ammonia synthesis section to the carbamate synthesis section;
providing means for feeding at least part of a flow comprising carbamate in aqueous solution coming from the urea recovery section to the carbamate decomposition section;
providing means for feeding a flow comprising ammonia and carbon dioxide in vapour phase obtained in the decomposition section to the urea synthesis section;
providing means for feeding a flow comprising diluted carbamate in aqueous solution obtained in the carbamate decomposition section to the carbamate synthesis section;
providing means for feeding a gas flow comprising hydrogen and nitrogen obtained in the carbamate synthesis section to the ammonia synthesis section;
providing means for feeding a flow comprising carbamate in aqueous solution obtained in the carbamate synthesis section to the carbamate decomposition section and/or to urea synthesis section.
Advantageously, the present invention allows to increase remarkably the production capacity of the ammonia, respectively urea plant, realising in a simple and effective way a partial integration between the two plants and accordingly achieving a debottlenecking of the existing apparatuses upstream of the respective synthesis sections which are substantially not involved in the capacity increase.
In other words, thanks to the step of providing additional sections of carbamate synthesis and decomposition suitably connected to the existing sections of the ammonia and urea plants, it is possible to increase the production capacity of such plants without being forced to increase the flow rate of the reactants flowing through the decarbonation, methanation and compression sections that may then operate optimally.
In fact, the amount of reactants necessary for obtaining the desired increase of production capacity of ammonia and urea may advantageously be produced in the additional sections of carbamate synthesis and decomposition that are independent from the existing sections of decarbonation, methanation and compression.
In particular, as it will appear more clearly in the following description with reference to the drawings, thanks to the carbamate synthesis section, a gas flow is obtained which comprises hydrogen and nitrogen as additional reactants for ammonia synthesis, whereas thanks to the carbamate decomposition section a gas flow is obtained comprising ammonia and carbon dioxide as additional reactants for urea synthesis.
A further advantage resulting from the method of modernization according to the present invention is given by the fact that, by providing the additional carbamate decomposition section for submitting to a treatment of partial decomposition at least one portion of the carbamate flow in aqueous solution resulting from the urea recovery section, it is possible to supply to the carbamate synthesis section a solution with a high content of water and at the same time to supply to the urea synthesis reactor a flow comprising ammonia and substantially anhydrous carbon dioxide, that permits to decrease the molar ratio H2O/CO2 in such reactor and therefore to increase the conversion yield of urea.
Accordingly, this feature is advantageous, not only because of the increase of production capacity of the urea plant but also because of the remarkable reduction of the energy consumption together with the increase of the conversion yield.
Further on, in doing so, it is possible not only to maintain a low H2O/CO2 molar ratio in the urea synthesis reactor, but also to exploit advantageously at least part of the water contained in the carbamate flow in aqueous solution coming out from the urea recovery section, recycling it in an easy and economic way to the carbamate synthesis section in order to enhance the absorption of the carbon dioxide, so as to obtain and maintain the carbamate produced in aqueous solution, thus avoiding an undesired crystallization of the same.
According to a further aspect thereof, the present invention relates to a process for the combined production of ammonia and urea in a plant of the type comprising a section for the production of raw ammonia synthesis gas comprising carbon monoxide, hydrogen and nitrogen, a carbon monoxide conversion section, a decarbonation section, a methanation section, a compression section for the ammonia synthesis gas, an ammonia synthesis section, a carbon dioxide compression section, a urea synthesis section and a urea recovery section, the process being characterised in that a first portion of ammonia and urea is produced through the steps of:
flowing a flow of raw ammonia synthesis gas comprising carbon dioxide, hydrogen and nitrogen through the sections of decarbonation, methanation and compression of the synthesis gas, obtaining a suitably compressed gas flow comprising hydrogen and nitrogen;
feeding the suitably compressed gas flow comprising hydrogen and nitrogen to the ammonia synthesis section;
feeding a portion of the ammonia obtained in the ammonia synthesis section together with the carbon dioxide coming out from the decarbonation section to the urea synthesis section;
whereas a second portion of ammonia and urea is produced through the steps of:
submitting at least part of a flow comprising carbamate in aqueous solution coming from the urea recovery section to a treatment of partial decomposition in a carbamate decomposition section, thus obtaining a flow comprising ammonia and carbon dioxide in vapour phase and a flow comprising diluted carbamate in aqueous solution;
feeding the flow comprising ammonia and carbon dioxide in vapour phase to the urea synthesis section;
feeding the flow comprising diluted carbamate in aqueous solution resulting from the treatment step to a carbamate synthesis section;
feeding a suitably compressed raw ammonia synthesis gas flow comprising carbon dioxide, hydrogen and nitrogen to the carbamate synthesis section;
feeding a portion of a flow comprising ammonia, hydrogen and nitrogen obtained in the ammonia synthesis section to the carbamate synthesis section;
reacting ammonia with carbon dioxide in the carbamate synthesis section, obtaining a flow comprising carbamate in aqueous solution and a gas flow comprising hydrogen and nitrogen;
feeding the flow comprising carbamate in aqueous solution to the carbamate decomposition section and/or to the urea synthesis section;
feeding the gas flow comprising hydrogen and nitrogen to the ammonia synthesis section.
The present invention relates also to a plant intended for implementing the aforesaid process for the combined production of ammonia and urea that is characterised in that it comprises:
a section for the production of raw ammonia synthesis gas comprising carbon monoxide, hydrogen and nitrogen, a carbon monoxide conversion section, a decarbonation section, a methanation section, a compression section of ammonia synthesis gas, an ammonia synthesis section, a carbon dioxide compression section, a urea synthesis section, a urea recovery section, a carbamate synthesis section and a carbamate decomposition section;
means for feeding a suitably compressed raw ammonia synthesis gas flow comprising carbon dioxide, hydrogen and nitrogen to the carbamate synthesis section;
means for feeding a portion of a flow comprising ammonia, hydrogen and nitrogen obtained in the ammonia synthesis section to the carbamate synthesis section;
means for feeding at least part of a flow comprising carbamate in aqueous solution coming from the urea recovery section to the carbamate decomposition section;
means for feeding a flow comprising ammonia and carbon dioxide in vapour phase obtained in the decomposition section to the urea synthesis section;
means for feeding a flow comprising diluted carbamate in aqueous solution obtained in the carbamate decomposition section to the carbamate synthesis section;
means for feeding a gas flow comprising hydrogen and nitrogen obtained in the carbamate synthesis section to the ammonia synthesis section;
means for feeding a flow comprising carbamate in aqueous solution obtained in the carbamate synthesis section to the carbamate decomposition section and/or to urea synthesis section.
According to the invention, the plants intended for implementing the process for the simultaneous production of ammonia and urea can be realised both ex-novo andxe2x80x94preferablyxe2x80x94by modifying existing plants so as to obtain an increase of the production capacity thereof and an improved performance with respect to the energy consumption.
Further features and advantages of the present invention will appear more clearly from the following non limitative description of an embodiment of the method of modernisation, respectively of the urea synthesis process according to the invention, made with reference to the attached drawings.